This section is intended to introduce various aspects of the art, which may be associated with embodiments of the disclosed techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the disclosed techniques. Accordingly, it should be understood that this section is to be read in this light, and not necessarily as admissions of prior art.
Loading and unloading of liquefied natural gas (LNG) carriers during transport can be disrupted by sloshing of the LNG cargo. A typical LNG delivery chain includes:                gas production from underground reservoirs,        gas processing/treating to remove heavier hydrocarbons and undesirable components, such as hydrogen sulfide and carbon dioxide,        a liquefaction plant to refrigerate the natural gas to a liquid state for storage and transport,        an export terminal facility, typically a harbor with berths for LNG carriers,        LNG carrier ships for marine transportation of the LNG from the export terminal to the market location, and        an import terminal at the market location to receive the LNG from the LNG carrier ships and vaporize the LNG into natural gas to be transmitted to the market by pipeline.        
In order minimize onshore impacts, enter new sales markets or capture new gas resources, there has been a desire to utilize offshore LNG terminal concepts. These concepts may include the use of floating units (floating production/liquefaction/storage for export and floating storage/regasification for import) or fixed units such as Gravity Based Structures (GBS). These offshore terminals typically require the LNG carrier to be exposed to the environment (specifically waves) during cargo operations (loading and unloading). In the past, cargo operations occurred in sheltered harbors where the environmental conditions are not significant.
A factor in evaluating the feasibility of offshore LNG export and import terminals is the terminal's berth availability. In regards to an offshore LNG terminal, availability is defined as the percentage of time that, on average, a LNG carrier can safely perform cargo operations (loading or unloading). These operational limits are established by the capacities of the mooring system, loading/offloading systems and the capacity of the LNG carrier's cargo containment system (CCS) to withstand cargo sloshing loads due to ship motions resulting from the weather and wave conditions at the site.
During cargo operations, the LNG Carrier tanks will be in partially-filled conditions as they are loaded or unloaded. As with any liquid, a partially filled tank allows for motion of the liquid inside the tank when the tank is excited (moved). This motion can cause the liquid cargo to impact the tank which can result in large forces and pressures on the tank. If the capacity of the tank to resist these impacts is lower than the resulting forces and pressures, damage to the tank may occur.
LNG cargo sloshing has the potential to damage the CCS aboard LNG carriers. Vessel motions drive cargo sloshing, which results in pressures and loads on the cargo tanks The LNG carrier motions are mainly driven by the LNG carrier design, LNG carrier loading condition (cargo tank filling levels, weight/displacement, centers of gravity, etc.) and the wave conditions (wave direction relative to LNG carrier, wave period and wave height).
Because the majority of LNG cargo operations have historically taken place in sheltered harbors, the impact of partial fill operations and the environment (waves) has been limited. With the need to perform cargo operations increasing for exposed/offshore berths, the potential for partial fill sloshing damage will be a key issue in determining terminal operability limits and project feasibility/economics. Current analysis highlights the challenges of achieving acceptable levels of availability with industry standard ships at offshore terminals. Reduction of sloshing loads and/or increased CCS structural capacity may help to meet availability requirements.
The ability to control the heading of a floating terminal is one technique to reduce the impact of cargo sloshing. In systems that provide this capability, the heading of the LNG Carrier may be altered with respect to the waves. This prevents the LNG Carrier from being exposed to beam seas and/or to allow the terminal to provide shelter. However, fixed terminals are not able to be positioned relative to waves. Additionally, repositioning is not as effective when the direction of the wind driven and swell waves are greatly separated.
Another potential strategy for dealing with cargo sloshing is the use of dedicated (i.e., purpose-built) LNG carriers. Dedicated LNG carriers may be built specifically for use at offshore terminals. Such dedicated LNG carriers may include strengthened membranes, subdivided membrane tanks, a MOSS containment system or an IHI SPB containment system. However, the strengthened membrane may not increase capacity enough to significantly improve operability. Requiring purpose built ships may reduce the marketing and commercial flexibility of the terminal by eliminating some industry standard ships from consideration for spot or short term charters.